Smokeless slow burning cast propellant

ABSTRACT

1. A cast, combustible composition comprising: granular ammonium nitrate; catalytic amounts of a powdered combustion catalyst intimately mixed with the ammonium nitrate, said catalyst being selected from the group consisting of zirconium hydride, cobalt, nickel, alloys of cobalt, nickel, and zirconium, and the oxides of cobalt and nickel; and an elastic, combustible, crosslinked, resinous fuel consisting of a copolymer which is preponderantly methyl acrylate in sufficient amount to fill the voids between the granules of ammonium nitrate and catalyst.

This invention relates to improvements in smokeless combustible productssuitable for use in jet propulsion motors, gas turbines, and similardevices.

A solid combustible composition suitable for use in jet propulsionmotors, gas turbines, and similar devices should be a slowly combustiblematerial, whose rate of oxidation is slow compared to an explosion. Allof the oxygen necessary for the combustion should be contained withinthe solid composition. It is well established that ammonium nitrate isuseful as an explosive, when the detonation is initiated by a highvelocity booster explosive. Much effort has been expended by manyinventors to utilize ammonium nitrate as an oxidizer in solid propellantcompositions suitable for jet propulsion devices. Previously disclosedcompositions containing ammonium nitrate as the sole oxidizer, incombination with a fuel, have failed to burn in a sustained andreproducible manner. Previously disclosed compositions containingammonium nitrate as the sole oxidizer, or as a substantial percentage ofthe total oxidizer have cracked on exposure to successively varyingtemperatures. This cracking phenomenon is due to the crystalline volumetransitions of ammonium nitrate, which occur at -18°, 32.3°, 84.2°,125°, and 169° C.

The art of producing compact solid combustible compositions by theextrusion or pressing of gelatinized nitrocellulose is well known. Theproduction of compact solid propellant compositions without elaborateand expensive equipment is very desirable.

An object of this invention is to provide a smokeless solid propellantfor jet propulsion motors, gas turbines and similar reaction devices.

A second object of the invention is to devise oxidizers for compactsolid combustible compositions.

A third object of this invention is to provide a properly catalyzedammonium nitrate oxidizer for solid propellants.

A fourth object of this invention is to provide a method of castingcompact solid propellants.

Another object of this invention is to provide a solid propellant whichcan be used over a wide range of temperature conditions.

A further object of this invention is to provide a technique forproperly restricting surfaces of the propellant at which ignition shouldnot occur.

Other objects of this invention will become apparent in the followingdescription which is to be read in connection with the appendeddrawings, in which:

FIG. 1 is a diagrammatical sectional view of a mold and liner therein;

FIG. 2 is a similar view showing a liquid resin and oxidizer added; and

FIG. 3 is an enlarged fragmentary sectional view of a propellant chargeproduced by my method.

In my invention ammonium nitrate is thoroughly mixed with a catalyst inthe desired proportion and combined with the necessary amount of resinforming fuel composition. After polymerization of the resin former, thecasting is an elastic, nonporous solid, capable of withstanding rapidtemperature changes without cracking. The solid propellant charge can beburned in a jet motor chamber at a controlled and reproducible rate,without detonating.

I have found a class of catalysts useful in increasing the rate ofcombustion of the resinous fuel by the ammonium nitrate. This class ofcatalysts comprises powdered cobalt metal, nickel metal, zirconiumhydride, cobalt oxides and nickel oxides, and alloys of cobalt, nickeland zirconium.

EXAMPLE I

A typical preparation of the cast propellant containing ammonium nitrateas the oxidizer is as follows: 584 gm of ammonium nitrate and 16 gmpowdered cobalt metal catalyst were tumbled in a closed container atabout 40 rpm for 2 hours to mix them thoroughly. The dry powder wasadded slowly to an approximately equal weight of the resin formersolution already in the mold and was then stirred. A thin slurry wasformed which readily lost its occluded air after further stirring forabout 5 minutes. The slurry was allowed to settle for a few minutes, andthe excess resin former solution was removed with a filter leaf. Afterremoval of the supernatant liquid, there was a tendency for the filterleaf to remove an excess of monomer from the top of the casting. Whenthe mold was repeatedly pounded on a rubber pad, the liquid again flowedto the top of the charge, as a result of capillary action, and wasremoved by suction. Several short pounding periods, each followed byfiltering of the supernatant liquid, was required to reduce the monomercontent to the desired value of about 23 weight percent, i.e., 179 gm.The covered mold was then heated at 50° C. in a water bath for 40 hours.The greater part of the polymerization took place in a few hours, afteran induction period.

The resin former solution consisted of 80 parts monomeric methylacrylate, 10 parts monomeric diallyl maleate, and 10 parts maleicanhydride, catalyzed by 0.5 parts t-butyl hydroperoxide. A pure methylacrylate polymer is elastic and thermoplastic, readily cold-flowing. Onadding a crosslinking agent in relatively small amounts, such as diallylmaleate, the resulting polymer is still elastic, but like vulcanizedrubber it no longer cold-flows.

The maleic anhydride is added to the system containing methyl acrylateto repress a slight decomposition of the acrylate which takes place inthe presence of the cobalt. Alternatively, acid ranging in strength fromacetic acid to oxalic acid may be used. Maleic anhydride is preferredbecause it copolymerizes with the other monomers, becoming a part of theresin phase.

The resin formed in situ during the curing of the propellant serves as amechanical binder and as fuel for the oxidizer. An elastic resin isnecessary to permit the crystal volume changes to occur withoutdeveloping large strains in the casting. It is also desirable to have anelastic resin to minimize cracks that may be caused by launching shockand mechanical abuse of the propellant charge. The viscosity of theresin former should be sufficiently low to permit rapid escape of theair bubbles formed on pouring the solid into the solution. Thisimportant factor must not be overlooked, if the castings are to be freeof voids. All the common liquid resin monomers have sufficiently lowviscosities to permit rapid settling of the solid particles. Alow-viscosity monomer solution also minimizes the difficulty offiltering off the excess monomer.

Essentially the process of preparing the casting consists of filling thevoids between the crystals of oxidizer with fluid resin former, thenpolymerizing the liquid to an elastic resin, thus encasing every crystalwith an elastic sheath. Crystal volume changes can then occur withoutrupturing the resinous fuel bonding phase.

Other resins may be employed as binder and fuel for the oxidizer. Forexample, butyl methacrylate, vinyl acetate, styrene, and methylmethacrylate may be used in substitution for the methyl acrylate inwhole or in part. Other cross linking molecules can be substituted fordiallyl maleate, such as diallyl phthalate, diallyl oxalate, polyesterresin condensates of maleic acid and various glycols, and divinylbenzene.

The powdered metals and oxides should be of minimum particle size, inorder to introduce a maximum of catalyst area per unit of weight. Cobaltpowder whose particle size was 5 micron or less was used in Example I.Using cobalt whose particle size is 25 micron or less, it requires about4 weight percent catalyst instead of 2 weight percent to maintain thesame burning rate.

The following additional specific examples are given of slow burning,combustible, cast compositions of matter adaptable to production by mymethod.

                                      TABLE I                                     __________________________________________________________________________                                    Burning Rate                                  Sample                                                                            Composition (wt. %)         (in./sec)                                     No. NH.sub.4 NO.sub.3                                                                  Catalyst                                                                             Resin           1500 psia                                     __________________________________________________________________________    1   70.5 Co   6.4                                                                             methyl acrylate                                                                         70 pt                                                               diallyl maleate                                                                         20 pt                                                                            23.1                                                                             0.0856                                                        maleic anhydride                                                                        10 pt                                               2   70.5 Ni   5.8                                                                             Same as above                                                                              23.7                                                                             0.0725                                        3   69.5 Ni.sup.1                                                                           6.3                                                                             Same as above                                                                              24.2                                                                             0.0819                                        4   71.2 Zr--Co.sup.2                                                                       5.3                                                                             Same as above                                                                              23.5                                                                             0.0852                                        5   71.9 Zr--Ni.sup.3                                                                       4.4                                                                             Same as above                                                                              23.7                                                                             0.0792                                        6   67.3 Zr H.sub.2                                                                         9.4                                                                             Same as above                                                                              23.3                                                                             0.0880                                        7   75.0 Co.sub.2 O.sub.3                                                                   2.0                                                                             methyl acrylate                                                                         80 pt                                                               diallyl maleate                                                                         10 pt                                                                            23.0                                                                             0.107                                                         maleic anhydride                                                                        10 pt                                               8   74.6 Co   4.5                                                                             butyl methacrylate                                                                      90 pt                                                                            20.9                                                                             0.0615                                                        maleic anhydride                                                                        10 pt                                               9   75.3 Co   4.7                                                                             styrene   85 pt                                                               maleic acid-                                                                  propylene    20.0                                                                             0.0748                                                        glycol polyester                                                                        15 pt                                               __________________________________________________________________________     .sup.1 Activated Raney nickel                                                 .sup.2 50 wt % Zr and 50 wt % Co                                              .sup.3 50 wt % Zr and 50 wt % Ni                                         

The burning rates in the above table are measured on strands of the castpropellant enclosed in a nitrogen atmosphere at 1500 psia. The strandsare restricted to end burning combustion by the use of a suitablevarnish film.

Samples of the ammonium nitrate solid propellant castings have been heldfor a period of 24 hours at -40° F., then directly transferred to atemperature of 160° F. for a second period of 24 hours. One such set oftemperature conditioning periods constituted a cycle. The castings havebeen cycled at least six times, been sectioned, and examined formacroscopic cracks. No cracks were visible. Cycled charges have beenfired in test motors without any difficulty, further indicating theabsence of cracks in the propellant charges.

It is desirable to prevent some surfaces of the solid propellant fromburning, in order to maintain a predetermined burning area. I havediscovered an improved means and method of restricting the surface ofthese cast solid propellants against burning which utilizes woven glasscloth. An example of the method follows: Two layers of tightly wovenglass cloth are sewn into a cylinder 1 as shown in the drawings. One end2 is also closed by glass cloth. The cylinder is fitted into a flatbottom steel mold 3. The glass cloth form is best sewn on a mandrel ofsuch dimensions as to give a tight fit in the mold.

The monomer mixture 4 is then introduced and the cloth thoroughly wettedto displace the entrapped air. The oxidizer-catalyst mixture 5 is pouredin and stirred well. The subsequent process then follows the proceduresof Example I accounting for the additional weight of monomerimpregnating the glass cloth. All air is displaced from the glass clothliner, as any residual oxygen entrapped acts as a very effectiveinhibitor for the methyl acrylate polymerization, resulting in a poorlylaminated liner.

Since the monomer is very fluid, it readily penetrates the cloth, whilethe oxidizer particles remain inside the cloth liner 1. The cloth thusacts as a semipermeable membrane. As shown in FIG. 3, the resultingpolymerized casting or propellant charge comprises an outer layer 6 ofresin, a resin filled glass-cloth laminated intermediate portion 7, andan inner cast oxidizer-catalyst and resin mixture 8 all bonded togetherby the same resin.

Propellant charges constructed as specified above have been successfullyburned in a rocket motor, and the restrictive glass cloth liner haswithstood up to 4 minutes exposure to the heat and flame thus produced.Glass cloth has high tensile strength and the laminate protects thepropellant from mechanical abuse. Since the cloth is noncombustible, thedanger of partially decomposed portions clogging the nozzle on ejectionis minimized, for the casing remains intact.

An integrally cast restrictor of glass cloth may also be used with otheroxidizers and resin fuels. The same restrictor was used to preparehydrazine nitrate castings, using an 80% methyl acrylate--20% diallylmaleate resin as fuel. Other fuels may be used. For example, the moreviscous polymerizable polyester type of resin may be used to coat layersof the cloth which are then laid in the mold and held together on themold surface by their tackiness. The casting procedure is continued aspreviously outlined or a viscous mixture of oxidizer and polyester typefuel may be employed. The restrictant is then cured together with thepropellant, providing the resin impregnant in the cloth and the resin inthe oxidizer mixture copolymerize to form a satisfactory bond at theliner-propellant interface.

While the particular embodiment of the present invention has been shownand described, it will be obvious to those skilled in the art, thatchanges and modifications may be made without departing from thisinvention in its broader aspects, and therefore the aim in the abandonedclaims is to cover all such changes and modifications as fall in thetrue spirit and scope of this invention.

I claim:
 1. A cast, combustible composition comprising: granularammonium nitrate; catalytic amounts of a powdered combustion catalystintimately mixed with the ammonium nitrate, said catalyst being selectedfrom the group consisting of zirconium hydride, cobalt, nickel, alloysof cobalt, nickel, and zirconium, and the oxides of cobalt and nickel;and an elastic, combustible, crosslinked, resinous fuel consisting of acopolymer which is preponderantly methyl acrylate in sufficient amountto fill the voids between the granules of ammonium nitrate and catalyst.2. A cast, combustible composition comprising: granular ammoniumnitrate; catalytic amounts of a powdered combustion catalyst intimatelymixed with the ammonium nitrate, said catalyst being selected from thegroup consisting of cobalt, nickel, alloys of cobalt and nickel, andoxides of cobalt and nickel; and an elastic, combustible, crosslinked,resinous fuel consisting of a copolymer which is preponderantly methylacrylate in sufficient amount to fill the voids between the granules ofammonium nitrate and catalyst.
 3. A cast, combustible compositioncomprising: 65 to 80 parts of granular ammonium nitrate; 1 to 10 partsof a powdered combustion catalyst intimately mixed with the ammoniumnitrate, said catalyst being selected from the group consisting ofzirconium hydride, cobalt, nickel, alloys of cobalt, nickel, andzirconium, and oxides of cobalt and nickel; and 20 to 35 parts of anelastic, combustible, crosslinked, resinous fuel comprisingpredominantly a copolymer of 60 to 100 parts methyl acrylate.
 4. A cast,combustible composition comprising: 65 to 80 parts of granular ammoniumnitrate; 1 to 10 parts of a powdered combustion catalyst intimatelymixed with the ammonium nitrate, said catalyst being selected from thegroup consisting of cobalt, nickel, alloys of cobalt and nickel, and theoxides of cobalt and nickel; and 20 to 35 parts of an elastic,combustible, crosslinked, resinous fuel comprising pertinently acopolymer of 60 to 100 parts methyl acrylate.
 5. A cast, combustiblecomposition comprising: 65 to 80 parts of granular ammonium nitrate; 1to 10 parts of a powdered combustion catalyst intimately mixed with theammonium nitrate, said catalyst being selected from the group consistingof cobalt, nickel, alloys of cobalt and nickel, and the oxides of cobaltand nickel; and 20 to 35 parts of an elastic, combustible, resinous fuelconsisting of a copolymer of 60 to 100 parts of methyl acrylate, and 1to 20 parts of maleic anhydride.
 6. A method of casting in situ a solidpropellant composition, characterized by: providing a mold and internalliner therein, the liner being permeable to a resin forming solution;bringing together within said liner a mixture of granular oxidizer and aresin forming solution of low viscosity; stirring the mixture; allowingthe granular particles of oxidizer to settle, and the resin former topermeate said liner; removing excess resin former solution until theamount of resin former within said liner is approximately sufficient tofill the voids between the granular particles of oxidizer; andpolymerizing the resin former in situ to form an elastic, combustiblecasting.
 7. A method of preparing a cast, solid, combustible compositioncomprising: intimately mixing granular ammonium nitrate with catalyticamounts of a powdered combustion catalyst; further bringing together ina mold the mixture of ammonium nitrate and catalyst with a resin formingsolution of low viscosity, whose volume is sufficient to form astirrable slurry; stirring the whole; allowing the solid particles tosettle; removing excess resin forming solution until the amount of resinformer is approximately sufficient to fill the voids between the solidparticles; and polymerizing the resin former in situ to form an elasticcombustible casting.
 8. In the method of claim 7, the additional step ofinserting a glass-cloth, shaped container into the mold contiguously tosurfaces of the mold and previous to mixing components in the mold.
 9. Asolid propellant structure, comprising: a propellant body including agranular oxidizer and a polymerized resin; and a foraminous linerpermeable to said resin prior to polymerization and impregnatedtherewith but impermeable to said granular oxidizer.
 10. A solidpropellant structure, comprising: a propellant body including a granularoxidizer and a polymerized resin; a foraminous liner impregnated withsaid resin but free of said oxidizer; and a covering of resin on theexterior surface of said liner.